Your fish are beautiful! I recommend 2Gs or more but those sizes are fine. Just make sure you keep up with the water changes. A 1.5G should be cleaned 100% twice a week. Ammonia builds up very quickly in smaller tanks. Having so many shrimp in a small tank is overdoing it. In a tank that small you really shouldn't have any tank mates for your betta.

Indeed, a 1.5 gallon is fine as long as you maintain legit water conditions. Should probably bump up the number of water changes, though, and how much water you replace during them. Sorry, I can't throw out a suggestion because I don't have a tank of that size in use, but I'm sure someone will give some input.